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Flu season may be coming to an end in some parts of the world, but a new influenza virus harbored by birds may be starting its global assault.

On Tuesday, China reported four more cases of infection with the H7N9 influenza virus, a type of bird flu, in the eastern Jiangsu province. These cases follow the first three reports of the disease on Sunday, from Shanghai and Anhui province. The two Shanghai patients have died, and the Anhui patient is in critical condition.

Researchers are testing the flu strains isolated from the patients, and so far the World Health Organization (WHO) says the cases do not appear to be connected. According to Xinhua, 167 people have been in contact with the four most recently infected patients, but none of these individuals show symptoms of respiratory problems or fever, as the patients did when they fell ill. And so far, the WHO says there is no indication of human-to-human transmission.

Before these cases, scientists said there were no reports of people being infected with H7N9, a less virulent form of bird flu, so investigators are still tracing the patients’ whereabouts to figure out how they were infected. All the infected individuals were sick with fevers and other flulike symptoms before being diagnosed, and only one of the patients appears to have been in direct contact with birds.

The reports of new cases, which first bubbled up from the social-media network in China, could mean that the Chinese authorities are getting a better handle on the virus. “When you don’t look, you don’t find them, but when you look, you’ll find,” Dr. Ray Yip, a public-health expert who heads the Bill & Melinda Gates Foundation in China, told the Associated Press. “A lot of people get severe respiratory conditions, pneumonias, so you usually don’t test them. Now all of a sudden you get this new reported strain of flu and so people are going to submit more samples to test, [so] you’re more likely to see more cases.”

Most forms of avian influenza do not infect humans, with notable exceptions, like the H5N1 strain that has claimed over 300 lives worldwide after an outbreak spread from infected poultry in Asia in 2003. But even H5N1 didn’t pass easily from one person to another, and the majority of people who fell ill had contact with contaminated poultry. However, two groups of scientists have successfully demonstrated how H5N1 can be genetically modified to transmit easily between ferrets, suggesting human transmission could also be possible after several mutations of the virus. Their findings led to an unprecedented call from a government safety group to halt their research, out of fear the publication of the work could be used in bioterrorism. The scientists recently asked to continue the work, arguing that only by understanding and studying how the virus can jump from one mammal to another can they find more effective ways of preventing the virus from spreading and causing a pandemic.

The Chinese health bureau said it is monitoring suspicious cases of the flu, and local health officials have created teams of experts to research the virus. Officials at schools and hospitals, where cases of flu spread more quickly, are on the lookout for fevers and flulike symptoms including respiratory problems.

We spoke to Gregory Hartl, the news coordinator in the director general’s office of WHO, to understand how the new strain differs from previous bird-flu viruses and how health officials are working to prevent a major outbreak.

What is the difference between this strain of the bird flu and H5N1?Influenza viruses are characterized by the hemagglutinin (H) and neuraminidase (N) proteins on the virus. They can have up to 17 hemagglutinin proteins and 10 neuraminidase proteins, and you can have any combination of that. For example, the bird-flu virus that we have known previously is called H5N1, so hemagglutinin 5 neuraminidase 1. This new virus has a new protein structure that is called H7N9.

How concerning are these new cases?We are taking it very seriously, and it certainly is serious because they are the first cases ever recorded in humans of H7N9. Something has allowed these cases to occur in humans, so there must have been a change somewhere in the structure of the virus that makes it easier to infect humans. We need to figure out what that is. We need to figure out what the host of the virus is in order to try to combat it and prevent infections as much as possible.

Are the symptoms of H7N9 similar to H5N1?Yes, in general they are pretty similar. This starts with a high fever and moves into pneumonia. This is similar to what happens in H5N1. Flu symptoms are generally upper-respiratory symptoms in mild cases, and cases are severe when it descends into the lung.

What is being done to track down these cases and the source?
There is a lot being done. The China CDC is very active and really enthused in doing all the medical work on this. They are investigating various animal sources and are trying to determine the extent of the spread of this virus because it is in two to three different locations in China. There are still a lot of holes in our knowledge. We don’t know what the source is. We don’t know where people got the strain, and we don’t know if it is transmitted from animals to humans or humans to humans. We don’t know.